Contaminated soils and residues, and ground waters draining these soils or residues pose an environmental problem to our industrial societies. Excavating, ploughing and covering contaminated soils or residues as well as pumping contaminated waters for further treatment in treating plants have been used in the past to solve this problem. These procedures however take time and effort and are usually practised at high costs.
Methods of in situ decontamination have been proposed, which methods address to this problem with more practical and less expensive solutions.
A great deal of work has been devoted for remediation of soils containing volatile contaminants. These technologies make use of means for injecting air in soils, which air may be heated if necessary. Contaminants having affinity for gaseous fluids are carried thereby and are collected by vacuum at the same level by immediate suction or at a higher level if air is injected in a water saturated zone. The water is in this case used as a medium allowing for proper ascending movement of the air-conveyed contaminants. When the environment wherein air is injected is not water saturated, a hydraulic gradient may also be created to govern the direction of air-conveyed contaminants. These technologies are described in U.S. Pat. Nos. 4,832,122, 5,193,934, 5,221,159, 5,244,310, 5,271,693, 5,389,267, 5,395,950, 5,403,119 and 7,935,950.
U.S. Pat. No. 5,398,756 and international patent publication WO 95/01232 make use of an electric current applied to direct the movement of ionic contaminants towards a treating zone. The environment may be saturated with water or a water gradient may be applied to attend the proper direction of the contaminants.
Other methods of in situ remediation are disclosed in international patent publications WO 95/00208, 94/25191, 94/23857, U.S. Pat. Nos. 4,850,745, 5,302,286 and 5,316,751. These methods are directed to water soluble or suspendible contaminants. These methods make use of water injection to completely immerse the contaminated site or residues or of water injection pumped over the contaminated site or water drawn from a saturated upper level (vadose) that further percolates through the site to a collecting system installed to a lower level. Decontaminating agents like bacteria may be added in the injected water or may be located in proximity of the collecting system which becomes a treating zone. When indigenous bacteria are put to contribution for remediation, the decontaminating agents may be nutrients and/or oxygen to support the growth and activity of the microorganisms.
All these references describe methods that are directed to a specific category of contaminants, volatile or water soluble. When water soluble contaminants are targeted, these methods do not encourage a long residence time of water in the soils to provide a saturated water site propitious for an extensive action of the decontaminating agents, unless the site is completely immersed or contaminated water is pumped over a site of low porosity (which porosity however may not be homogenous) and collected towards a treating zone or reactor. Contaminated water pumping has the disadvantage that contaminants may not be transported efficiently to the treating zone, and a substantial part thereof remains in the site, or multiple injecting, percolating, collecting and recirculating steps are necessary to efficiently carry contaminants to the treating zone. If plants are to be grown on the site for esthetic purpose or put to contribution as decontaminating agents (because they can hyperaccumulate metals and radioactive substances), a suitable level of water cannot be carefully controlled and maintained to encourage their optimal growth and activity. Users are left with only one choice: growing aquatic plants that may grow in a completely immersed site, if feasible. When a non-immersion technique is used, injection and percolation of water may carry contaminants away from the decontaminating plants while not providing optimal conditions of growth and activity.
There is clearly a need for a more performing and more versatile method of remediation that could be achieved if the level of the water table in the site is carefully controlled.